Most handheld consumer electronic devices, such as wireless telephones, notebook computers, and personal digital assistants (PDAs) are powered either by rechargeable batteries or by disposable batteries.
In the area of rechargeable batteries, historically nickel-cadmium batteries were used. More recently nickel-metal-hydride has been used and still more recently lithium-ion technology has been used. These shifts in battery chemistry have improved the power-to-weight ratio but nature imposes upper bounds on the energy density available in rechargeable batteries.
In the area of disposable batteries the chief technology employed presently is alkaline cells. Nature also imposes upper bounds on the available energy density for such cells.
Those who design handheld consumer electronic devices are thus faced with limits on battery life imposed in part by a desire to keep the devices from getting too heavy and large.
In recent years much attention has been paid to the prospect of employing fuel cells in a variety of applications including the powering of handheld consumer electronic devices. It seems possible that after various challenges are overcome, fuel cells may prove to be a useful power source for such applications. Fuel cells offer the possibility of a light-weight power source using inexpensive fuel, with fuel that is easy to refill.
There are, however, a number of challenges with present-day fuel cells. They run down. Refilling them can be a bother. It is not easy to extract all available energy from a given charge of fuel.
One fuel cell is called a direct methanol fuel cell (DMFC). In a DMFC, methanol is reacted with oxygen, one byproduct of which is water. As shown in FIG. 3, the methanol and oxygen flow toward a proton exchange membrane. Electrical power is derived from an anode and cathode juxtaposed with the membrane. Importantly, the “active fuel” area 2 in FIG. 3 is not filled with pure methanol but instead contains a solution of methanol in water. A typical methanol concentration is 3%.
DFMCs generate electricity through decomposition of methanol into hydrogen ions and electrons. Hydrogen ions propagate through the proton exchange membrane into the cathode area, while electrons reach the cathode through a load providing electricity in the process. Electrons reaching the cathode area recombine with hydrogen ions which in turn combine with oxygen supplied by the air to provide pure water as a byproduct.
Many investigators have attempted to devise suitable fuel cell structures, as shown for example in U.S. published application publication No. 20040001989 entitled “Fuel reservoir for liquid fuel cells” and publication No. 20020127141 entitled “Multiple-walled fuel container and delivery system.” See also published international application with publication No. WO 03/094318 entitled “Device and method to expand operating range of a fuel cell stack.”
As mentioned above, DMFCs are comparatively small and potentially suitable to be used in small electronic appliances. Key for such an adoption is further miniaturization, reduction in cost as well as improvements in the performance of the cell.
It would be extremely desirable to devise ways to improve DMFCs to perform better.